The present invention relates to the measurement of magnetic flux in laminated cores, and more particularly to a flux sensing means which can be built into a laminated core such as the stator core of a large turbine generator to determine the magnetic flux in regions of the core not otherwise accessible.
In large synchronous dynamoelectric machines, the currents in the end turn portions of the rotor winding and in the end portions of the stator windings have magnetic fields which combine to produce an axially-directed magnetic flux. This axial flux enters the end of the stator core in a direction generally perpendicular to the core laminations, and causes relatively large eddy currents in the end regions of the core. The corresponding losses can be quite high and often result in excessive heating in the end regions of the core. As the maximum ratings of large turbine generators have increased, this has become an increasingly severe problem because of the high iron temperature in the end regions of the core, particularly in the tooth areas, and has sometimes resulted in undesirable limitations on the ratings of large machines. Calculation or prediction of the localized temperatures in the end portion of the stator core is a very complex problem because the axial flux interacts with the armature leakage flux from the stator coils, as well as with the radial airgap flux, resulting in high saturation levels in the stator tooth region. The interaction of these various fluxes at any particular point and time depends on their relative magnitudes and phase relations, which are time varying and difficult to determine with any accuracy. The temperature profile axially of the stator core is dependent on the axial penetration depth of the axial flux, as well as on the relationship between the localized loss and the axial component of the flux at each point. All these varying and complex factors make it extremely difficult to predict the temperature rise in the end region of the stator core of a large generator. It is highly desirable, therefore, to make accurate measurements of the flux actually existing at particular points in the core, so that the magnitude and distribution of the axial flux can be determined. It has not been possible heretofore, however, to make such measurements because there is no way in which conventional search coils could be inserted into a laminated stator core at the desired locations without the coils being destroyed by the heavy pressure under which the core laminations are clamped, and without affecting or disrupting the structural integrity of the stator core itself.